Allenes, /3-hydride elimination

The proposed mechanism involves coordination of allene and ce,/j-unsaturated ketone to the cationic cydopentadienylruthenium species 137. Subsequent formation of the ruthenacyde 139, followed by /3-hydride elimination, generates the ruthenium hydride species 140. Finally, reductive elimination closes the cycle and regenerates the ruthenium intermediate 137 (Scheme 14.33) [68, 71]. 

A rhodium-catalyzed allenic Alder ene reaction effectively provides cross-conjugated trienes in very good yields (Scheme 16.70) [77]. The reaction most likely involves ft -hydride elimination of an intermediate rhodium metallacycle to afford an appending olefin and ensuing reductive elimination of a metallohydride species to give the exocyclic olefin. 

The isomerization of alkylzirconocenes proceeds by a series of (3-hydride eliminations and insertions. Because the C(sp2)-Zr bond is much stronger than the C(sp3)-Zr bond, and because the allene product that would be generated by (3-hydride elimination from an alkenylzirconocene is high in energy, the 13-hydride elimination is uphill in energy. 

The mechanism proposed for this allenic Alder-ene reaction is shown in Scheme 8.3. The rhodium] I) catalyst coordinates with the allenyne V forming intermediate VI, which undergoes an oxidative addition to form the metaUocycle VII. The metaUocycle then undergoes a /9-hydride elimination producing triene intermediate VIII, which... 

The allylic sulfonyl allene 35, possessing unsymmetrical substitution at the distal double bond of the allene, was examined. Since there are two possible sites for y5-hy-dride eliminahon, a variety of isomers are possible. Treatment of allene 35 with [Rh(CO)2Cl]2 gave a 3 5 1 raho of the three possible products 36, 37, and 38, which translates into an 8 1 constitutional group selectivity in the y5-hydride elimination step ((364-37) 38) and an T/Z-selechvity of 1 2 (Tab. 8.8, entry 1). 

In contrast to the efficient reactions illustrated above, the use of 1,2-disubstituted aikenes as the 2n -components in the [5-1-2] cycloaddition has resulted, thus far, in low cycloadduct yields and complex mixtures, putatively arising from an intermediate metal-lacycle through competitive yS-hydride elimination. This limits access to the carbocyclic cores of some large and medicinally interesting natural product families (for example, those in Scheme 13.3). Introduction of an allene substrate, however, circumvents this limitation by installing the needed carbon-carbon bond while simultaneously leaving a handle for further functionalization (Scheme 13.10). For example, reduction of the exo-... 

Under palladium catalysis, the < -iodoallenylbenzene 146 first undergoes an intermolecular carbopalladation of the double bond in the added norbornene 116, and only then follows an intramolecular carbopalladation of the allene moiety in 146 ensuing /3-hydride elimination finally provides the tricyclic compound 147 (Scheme 37)7 ... 

A ruthenacyclopentane 48 has been proposed as an intermediate in this reaction, after coordination of the allene and enone. Exocyclic /1-hydride elimination led to the 1,3-dienes. This ruthenacycle possessed a o-bound ruthenium allyl, allowing nucleophilic additions by alcohols or amines. Alkylative cycloetherification [29] (Eq. 20) and synthesis of pyrrolidine and piperidine [30] were thus achieved. 

Cyclopropyl-substituted allenes open the door to yet another reaction mode. When treated with aryl iodides in the presence of a typical Heck-catalyst system and a dienophile, cyclohexene derivatives 77 were obtained (Scheme 11) [53,54]. Thus, the initially formed arylpalladium iodide car-bopalladates 72 to form a a-allylpalladium intermediate 73. It swiftly undergoes the cyclopropylcarbinyl to homoallyl rearrangement yielding the ho-moallylpalladium species 74 which finally suffers /1-hydride elimination. The thus formed 2-aryl-1,3,5-hexatrienes 75 are prone to undergo polymerization, but can be efficiently trapped by an appropriate dienophile at the least steri-... 

B. INTERMOLECULAR CARBOPALLADATION OF ALLENES FOLLOWED BY /8-HYDRIDE ELIMINATION REACTION... 

Shimizu and Tsuji reported the hrst catalytic carbopalladation reaction of allenes in 1984. The reaction of Phi with 1,2-heptadiene in the presence of Pd(OAc)2, dppe, and N-methylpyrrolidine (NMP) as the base gave 2-phenyl-l,3-heptadiene 18 (Scheme 7) via the /3-hydride elimination from the 7r-allylpalladium intermediate 17. ... 

As outlined in Sect. B and C, catalytic intermolecular carbopalladations of allenes followed by either /3-hydride elimination or intermolecular nucleophilic trapping provide 1,3-dienes or allyl derivatives bearing the nucleophile moiety, respectively, while an intermolecular carbopalladation followed by intramolecular trapping sequential reaction provides cyclic skeletons (Scheme 27). In Type I, the nucleophilic moiety is connected with the C—X bond, and in lype II it is attached to the allene moiety. 

The RhI-catalysed rearrangement of A-allylaziridines to (Z)-A-alkenylaziridines has been investigated by computational methods and the reaction occurs via a hydrometalation//3-hydride elimination. Alkylidene cycloheptadienes have been prepared via a rhodium(I)-catalysed tandem isomerization of cyclopropylenynes through a [1,5] carbon-carbon migration via an allene intermediate with high Z E selectivities (Scheme 152). ... 

The proposed mechanism is shown in (Scheme 43). Based on the results of allenic Pauson-Khand type reaction (PKTR), it is known that the Rh catalyst preferentially coordinates to the distal double bond of the allene-forming metallacycle I-I and sequential CO insertion would provide [2-I-2-1-1] product. However, in the absence of CO /3-hydride elimination takes place instead leading to triene intermediate I-II, and subsequent reductive elimination affords the cross-conjugated triene 83. 

This same palladium jt-addity can be employed to activate alkenes towards intramolecular cydization with heteroatom-hydrogen bonds. In contrast to alkynes or allenes, the addition of N H, O—H and other heteroatom-hydrogen bonds across the alkene would formally create a saturated carbon-carbon bond, rather than the unsaturation necessary for aromatic heterocydes. As such, subsequent P-hydride elimination is often required to dired these readions towards aromatic produds, in a Wacker-type alkene oxidation (Scheme 6.28). 

Sam B, Luong T, Krische MI (2015) Ruthenium-catalyzed C-C coupling of fluorinated alcohols with allenes dehydrogenation at the energetic limit of p-hydride elimination. Angew Chem Int Ed 54 5465-5469... 

In Sect B, the intermolecular carbopalladations of allenes are followed by a /3-hydride elimination to afford a 1,3-diene. However, in the presence of a nucleophile, a Tsuji-Trost-type nucleophihc substitution of the intermediate 7r-allylpalladium spedes has been fully estabhshed (Scheme 6). 

To a hydridopalladium species J generated from Pd(PPh3)4 and benzoic acid, hydropalladation of alkyne 20 and subsequent jS-hydride elimination occur to give allene 21 and to regenerate the active catalyst J (cycle I). Subsequent hydropalladation of 21 with J affords the q -allyl palladium K, and external nucleophilic attack gives hydroamination product 23 (cycle II). Intramolecular version is also documented in Sect. 4.1. 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

These results support the /3-elimination from 220 to give 221, towards which KOtBu acts as a base and a nucleophile. As in the case of 215, the addition occurs at the central allene carbon atom leading the allyl anion 222, which is protonated to yield 223. On the other hand, the deprotonation of the methylene group brings about 224, whose major amount is converted to naphthalene, but a small proportion, behaving as a nucleophile, traps 221, giving rise to the allylanion 225, which in turn reacts with 221 and, by a hydride transfer, furnishes 228 and the allyl anion 229. By protonation, the latter is converted into 226. By conducting this experiment in the presence of benzophenone, this mechanistic model was confirmed as the tertiary alcohols 227 and 230 were obtained in addition to naphthalene, 223 and 228. Apparently, the anions 224 and 229 were intercepted in part or totally, respectively, by benzophenone (Scheme 6.52) [137]. 

The nucleophilic attack on an acceptor-substituted allene can also take place at the acceptor itself, especially in the case of carbonyl groups of aldehydes, ketones or esters. Allenic esters are reduced to the corresponding primary alcohols by means of diisobutylaluminum hydride [18] and the synthesis of a vinylallene (allenene) by Peterson olefination of an allenyl ketone has also been reported [172]. The nucleophilic attack of allenylboranes 189 on butadienals 188 was investigated intensively by Wang and co-workers (Scheme 7.31) [184, 203, 248, 249]. The stereochemistry of the obtained secondary alcohol 190 depends on the substitution pattern. Fortunately, the synthesis of the desired Z-configured hepta-l,2,4-trien-6-ynes 191 is possible both by syn-elimination with the help of potassium hydride and by anti-elimination induced by sulfuric acid. Analogous allylboranes instead of the allenes 189 can be reacted also with the aldehydes 188 [250]. 

Condensation between the allenic aldehydes 25 and the allenylboranes 24, derived from the allenylsilanes 23, also exhibited high diastereoselectivity (Scheme 20.7) [33-35], However, unlike 17, a reversal of diastereoselectivity in favor of the RR/SS pair of the a-silyl alcohols 26 occurred. Consequently, treatment of 26 with potassium hydride to promote the syn elimination furnished the enyne-allenes 27 having predominantly the E configuration (fc Z>% 4) for the central carbon-carbon double... 

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